Field of the Invention
The present invention concerns a method to determine a B0 field map that describes the local deviation from a nominal Larmor frequency of a magnetic resonance device, wherein magnetic resonance data are acquired in measurements implemented at two different echo times whose difference forms a dephasing time, after an excitation at at least two different dephasing times; and a phase change to be used to determine the B0 field map is determined from a difference of phases measured at different echo times; wherein the phase changes of different dephasing times are evaluated for at least partially reducing an ambiguity due to Nyquist phase wrapping.
Description of the Prior Art
Magnetic resonance imaging and its basic operation are widely known in the prior art. A subject to be examined is introduced into a basic magnetic field with a relatively high field strength (known as the B0 field). In order to be able to acquire magnetic resonance data, for example in a slice of a subject, the nuclear spins of this slice are excited and the decay of this excitation is detected as a signal, for example. Gradient fields can be generated by a gradient coil arrangement while radio-frequency excitation pulses (which are frequently designated as radio-frequency pulses) are emitted via a radio-frequency coil arrangement. The entirety of the radio-frequency pulses (excitation) generates a radio-frequency field that is typically designated as a B1 field, and the spins of resonantly excited nuclei are deflected (with a spatial resolution due to the gradients) by an angle known as a flip angle relative to the magnetic field lines of the basic magnetic field. The excited spins of the nuclei then radiate radio-frequency signals that can be acquired and processed further by suitable reception antennas (such as by the radio-frequency coil arrangement itself) in order to thus be able to reconstruct magnetic resonance image data therefrom.
Conventional radio-frequency coil arrangements are operated in a mode known as the “homogeneous mode”, for example in a CP mode (circularly polarized mode), wherein a single radio-frequency pulse is emitted with a defined, fixed phase and amplitude to all components of the transmission coil, for example all transmission rods of a birdcage antenna. To increase flexibility and to achieve new degrees of freedom to improve the imaging, it has been proposed to also enable a mode known as a parallel transmission (pTX), in which multiple transmission channels of a radio-frequency coil arrangement are individually charged with individual pulses that can deviate from one another. This entirety of the individual pulses (which, for example, can be described via the parameters of phase and amplitude) is then defined as a whole in a control sequence (protocol) that is described by a corresponding parameter set. Such a multichannel pulse (excitation) that is composed of individual pulses for the different transmission channels is often designated as a “pTX pulse” (for “parallel transmission”). In addition to the generation of spatially selective excitations, field inhomogeneities can also be compensated (for example within the scope of “RF shimming”).
In order to determine control parameter sets of a control sequence, it is necessary to know the background (thus the B0 field), as well as the effects of the individual transmission channels in the imaging region (in particular the homogeneity volume).
For measurement of the basic magnetic field (B0 field)—designated as a B0 mapping—first magnetic resonance data are typically acquired (preferably by gradient echo imaging) at two different echo times. The phase difference (phase change) of the magnetic resonance data acquired at different echo times (which can be determined by subtraction of the phases of two magnetic resonance images of the first magnetic resonance data that are acquired at different echo times) is proportional to a deviation of the local B0 field from the nominal basic magnetic field strength and to the dephasing time (thus the difference of the two echo times). The field deviation is thereby specifically described by a deviation of the Larmor frequency from a nominal Larmor frequency of the magnetic resonance apparatus (a value describing this deviation is designated as a Larmor frequency value in the following).
The phase generated by deviations in the homogeneity of the B0 field thus develops over time. The effect of the Nyquist phase wrapping must be taken into account because the proportionality of the phase difference of magnetic resonance data acquired at different times to the deviation from the nominal Larmor frequency, and to the difference of the echo times, applies only as long as the phase difference (limited to 2π) corresponds to the actual phase evolution. However, the phases can be further developed by multiples of 2π depending on the dynamic range of the B0 distribution. This leads to ambiguities and errors in the calculation of the B0 maps. Incorrect associations in the phase evolution show themselves in non-physical spatial discontinuities due to the 2π jumps in the phase difference images. This means that an extremely fast development of the B0 phase also occurs if the deviation of the local Larmor frequency from the nominal Larmor frequency is high, such that the phase will go beyond 2π when the echo time (here the difference of the two echo times) is not short enough, such that the described ambiguity occurs.
The selection of extremely short dephasing times is frequently not possible due to the sequences that are used, and so smaller deviations from the nominal Larmor frequency can no longer be measured with sufficient precision given an extremely short echo time difference.
A few approaches are known in the prior art to solve the ambiguity problem in the association of the measured phase change. It is thus possible to choose the dephasing time (thus the difference of the echo times) to be so short that the phases do not develop by more than 2π at any location during them. However, since the dynamic range of the B0 field distribution is not known before the measurement, the dephasing time must be chosen to be so short that the sensitivity of the acquisition method is not sufficient, and this procedure is consequently not used (as has already been explained).
Therefore, it was proposed to detect and correct phase jumps in the B0 maps in a post-processing, under the assumption that the B0 field is spatially continuous. Algorithms that have this effect are designated as phase unwrapping algorithms. However, the reliability of such algorithms is often questionable. The primary difficulty exists in that the entire volume can be comprised of non-contiguous partial regions, such that individual partial regions of the B0 maps are separated by voxels that only include noise and are very low in signal. The phase in these voxels can thus not be determined, or can only be determined unreliably.
It has also been proposed to iteratively acquire first magnetic resonance data with increasing dephasing time, consequently increasing difference between the echo times. The shortest dephasing time is thereby selected so that no spatial phase jumps occur. Whether a phase jump will occur given longer dephasing time is estimated from the acquisitions with shorter dephasing times. If this is the case, this is taken into account in the evaluation (reconstruction) of the first magnetic resonance data with longer dephasing time. The phase ambiguity is therefore dispelled, and long dephasing times are enabled for a high sensitivity.
A further alternative procedure is to minimize the phase gradients between adjacent voxels in the B0 maps. Given this solution, the B0 maps do not necessarily need to be corrected for phase jumps. However, the risk exists that a calculated B0 shim is optimized for false B0 offsets in different spatial areas. Moreover, no frequency (zeroth order shim) can be calculated from differential method.
Corresponding mapping processes are also known for B1 fields and are designated as “B1 mapping”. Generally speaking, B1 field maps are acquired for each transmission channel, which means that the B1 field maps show how strong the B1 field is at a specific location in the imaging region given a specific excitation (for example a uniform excitation and/or given a defined transmitter voltage). This means that a complex B1 value (consequently a B1 amplitude and a B1 phase, which can also be differentiated in a B1 amplitude map and a B1 phase map) is associated with each voxel (image point). Typically, measurements for a number of excitation modes are conducted, wherein an excitation mode does not necessarily need to correspond to the operation of only one channel; rather, combinations are also conceivable from which the individual transmission channels can then be concluded.
In order to determine the amplitude of the B1 field, for example, it is known to measure the flip angle that a radio-frequency pulse causes, wherein reference is made to DE 10 2005 049 229 B3 as an example. An excitation mode thereby results in a constant phase shift (consequently a constant B1 field), wherein the B0 phase continuously varying over time (as it has been described) is, however, naturally also acquired as well in the phase measurement. Therefore, for B1 mapping it is known to use basically the same echo time for the different excitation modes, such that the effect of the B0 field on the phases is kept constant, such that a raw phase map obtained from the magnetic resonance data of one excitation mode can be used as a correction so that the stable effect of the B0 field on the phase cancels out, and consequently the phases of the excitation mode used for correction serve as a reference phase map. This means that all other B1 phase maps are defined relative to the B1 phase map used as a correction, which is, however, not problematic since it ultimately depends anyway only on the relative phases of the different transmission channels.